Problem: Express your answer in scientific notation. $7.4 \cdot 10^{-8} - 6.7 \cdot 10^{-9} = $
Explanation: $\phantom{=}{7.4 \cdot 10^{-8}} - 6.7 \cdot 10^{-9}$ $={74 \cdot 10^{-9}} - 6.7 \cdot 10^{-9}$ $= (74-6.7)\cdot10^{-9}$ $=67.3\cdot10^{-9}$ $=6.73\cdot10^{-8}$